The application of numerical models of free surface flow to fluvial flood prediction is currently hampered by the lack of distributed calibration and validation data. We address this shortcoming by the use of satellite-borne synthetic aperture radar imagery to map a flood on a 15-km reach of the river Thames, England. A finite element numerical model of shallow water flow is constructed over the reach, and analysis shows that floodplain friction is the dominant factor affecting inundation extent when compared to channel friction, turbulence parameterization, and downstream boundary conditions. The area of the domain correctly classified by the model is maximized with the constraint of uniform floodplain friction (77% at maximum), compared with 70% for a simple planar model of the water free surface. A simple distributed calibration scheme is also tested, with a minor improvement over the uniform parameterization. ¿ 2000 American Geophysical Union